Device packaging with substrates having embedded lines and metal defined pads

ABSTRACT

Package substrates enabling reduced bump pitches and package assemblies thereof. Surface-level metal features are embedded in a surface-level dielectric layer with surface finish protruding from a top surface of the surface-level dielectric for assembly, without solder resist, to an IC chip having soldered connection points. Package substrates are fabricated to enable multiple levels of trace routing with each trace routing level capable of reduced minimum trace width and spacing.

This is a Divisional of application Ser. No. 14/481,766 filed Sep. 9,2014 which is a Divisional of application Ser. No. 12/975,934 filed Dec.22, 2010 now U.S. Pat. No. 8,835,217 issued Sep. 16, 2014.

FIELD

Embodiments of the present invention relate to the semiconductor devicesand more particularly to semiconductor device packaging.

DISCUSSION OF RELATED ART

Integrated circuit (IC) devices, such as microprocessors are oftenpackaged with a chip mounted to a package substrate using a microballbump technology. Advances in the art continue to place a demand fordevice packaging capable of increased chip input/output (I/O) withreduced bump pitches. A conventional package substrate for microballtechnology is typically fabricated with a semi-additive process (SAP),as illustrated in FIG. 1.

As shown in FIG. 1, the SAP 100 first forms a dielectric build up layer104 over a metallized pad 102 formed in one of the layers of substrate101. A via hole 109 is then laser drilled in the dielectric build uplayer 104 and a metallic seed layer 115 is formed over the substrate. Atemporary resist pattern 120 is lithographically patterned over themetallic seed layer 115 and the conducting traces and vias 125 areelectroplated on the metallic seed layer 115. The temporary resistpattern 120 is then removed and the metallic seed layer 115 etched away.The SAP 100 continues in this manner to build up any number ofmetallization layers on a package substrate until a first (i.e. top)level metal is formed to which a solder ball is to be attached. Assumingthe conducting traces and vias 125 are the surface level metal, a solderresist film 140 is then formed over the conducting traces and vias 125.A solder resist film 140 is then lithographically patterned to form anopening over the metallized pad 136, and a surface finish 145 is appliedto the exposed portion of the metallized pad 136. A microball technologyis then used to place a solder ball 150 into the solder resist maskopening which is then reflowed.

At assembly, a chip connection point, such as a copper post formed aspart of a chip-side C4 process, is then aligned to the reflowed solderball 150, and another solder reflow is performed to join the chip to thesubstrate. An underfill and/or encapsulation process then completes theconventional process using microball technology.

As the bump pitch shrinks based on the demand for higher I/O routing,the package substrate line and spacing reduces accordingly, placingsignificant challenges on the current SAP to achieve high yields.Reduced bump pitches also place a demand on the current microballtechnology to achieve high backend yields (minimal losses due to bumpbridging and missing solder ball). To achieve these high yields atreduced bump pitch, the solder ball diameter should be lower leading toa lower bump height, which is a challenge for the assembly defluxprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a conventional semi-additivesubstrate fabrication process;

FIG. 2A is a flow diagram of a package substrate fabrication process forreduced bump pitch in accordance with an embodiment of the presentinvention;

FIG. 2B is a cross-sectional view of selected operations illustratingstages in the package substrate fabrication process described in FIG.2A, in accordance with an embodiment of the present invention;

FIG. 3A is a flow diagram of a package substrate fabrication process forreduced bump pitch in accordance with an embodiment of the presentinvention;

FIG. 3B is a cross-sectional view of selected operations illustratingstages in the package substrate fabrication process described in FIG.3A, in accordance with an embodiment of the present invention;

FIG. 4A is a flow diagram of a package substrate fabrication process forreduced bump pitch in accordance with an embodiment of the presentinvention;

FIG. 4B is a cross-sectional view of selected operations illustratingstages in the package substrate fabrication process described in FIG.4A, in accordance with an embodiment of the present invention;

FIG. 5 is a flow diagram of an assembly process utilizing a substratewith reduced bump pitch in accordance with an embodiment of the presentinvention;

FIGS. 6-7 are cross-sectional views of selected operations illustratingstages in the package assembly process described in FIG. 5 for asubstrate with reduced bump pitch, in accordance with an embodiment ofthe present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toprovide a thorough understanding of the present invention. It will beapparent to one skilled in the art that the present invention may bepracticed without these specific details. In other instances, well-knownfeatures, such as specific fabrication techniques, are not described indetail in order to not unnecessarily obscure the present invention.Reference throughout this specification to “an embodiment” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment is included in at least one embodiment ofthe invention. Thus, the appearances of the phrase “in an embodiment” invarious places throughout this specification are not necessarilyreferring to the same embodiment of the invention. Furthermore, theparticular features, structures, materials, or characteristics may becombined in any suitable manner in one or more embodiments. Also, it isto be understood that the various exemplary embodiments shown in theFigures are merely illustrative representations and are not necessarilydrawn to scale.

The terms “coupled” and “connected,” along with their derivatives, maybe used herein to describe structural relationships between components.It should be understood that these terms are not intended as synonymsfor each other. Rather, in particular embodiments, “connected” may beused to indicate that two or more elements are in direct physical orelectrical contact with each other. “Coupled” my be used to indicatethat two or more elements are in either direct or indirect (with otherintervening elements between them) physical or electrical contact witheach other, and/or that the two or more elements co-operate or interactwith each other (e.g., as in a cause an effect relationship).

The terms “over,” “under,” “between,” and “on” as used herein refer to arelative position of one material layer with respect to other materiallayers. As such, for example, one layer disposed over or under anotherlayer may be directly in contact with the other layer or may have one ormore intervening layers. Moreover, one layer disposed between two layersmay be directly in contact with the two layers or may have one or moreintervening layers. In contrast, a first layer “on” a second layer is incontact with that second layer. Additionally, the relative position ofone layer with respect to other layers is provided assuming operationsare performed relative to a substrate without consideration of theabsolute orientation of the substrate.

Described herein are methods of fabricating integrated circuit (IC) chippackage substrates which enable IC chip-substrate assemblies to beformed in a package with a reduced bump pitch. In certain embodiments, apackage substrate is fabricated to facilitate assembly which places asolder ball on the chip side and involves direct contact of the solderto a metal defined pad on the substrate rather than pad defined by asolder resist. As such, in certain such embodiments, no solder resist ispresent on the surface of the package substrate and no solder is placedon the package substrate prior to coupling a chip to the packagesubstrate. The fabrication processes described herein for an embeddedsurface metal feature may be repeated multiple times to provide routingon multiple layers. BHAST reliability issues that might exist wheresurface-level metal features are exposed to underfill (e.g., instead ofsolder resist) are mitigated by embedding the metal features in asurface-level dielectric film.

FIG. 2A is a flow diagram of a package substrate fabrication process 200for reduced bump pitch in accordance with an embodiment of the presentinvention. FIG. 2B provides cross-sectional views of selected operationsillustrating stages in the package substrate fabrication process 200, inaccordance with an exemplary embodiment.

Process 200 begins at operation 201 with lamination of a dielectriclayer (e.g., surface dielectric layer 110 in FIG. 2B) over a patternedmetal layer (e.g., sub-surface-level metal feature 105) in a substratebuild-up layer 101. Generally, the patterned metal layer and any numberof layers below the patterned metal layer may be formed in any mannerknown in the art. For example, the patterned metal layer may be a topbuild-up layer formed with an SAP similar to that illustrated in FIG. 1.The dielectric layer may be of any composition known in the art andapplied over the patterned sub-surface level metal layer in anyconventional manner. In a particular embodiment, the dielectric layer110 comprises a polymer (epoxy based resin) with silica filler toprovide suitable mechanical properties that meet reliabilityrequirements of the package. The material must also have a suitableablation rate to enable laser patterning as described elsewhere herein.

Returning to FIG. 2A, at operation 209 via holes (e.g., via holes 210 inFIG. 2B) are laser drilled into the dielectric layer (e.g., surfacedielectric layer 110) to expose a portion of the underlying metal layer.Any conventional technique may be used, such as one employing CO₂ laser.At operation 211 the dielectric is then further laser patterned to forma trace recess (e.g., trace recess 212 in FIG. 2B) and/or a pad recess(not depicted) with a depth in the dielectric that is less than that ofthe vias (i.e., the sub-surface level metal feature 105 is not exposedby the patterning at operation 211). As shown in FIG. 2B, the tracerecess 212 is laterally space apart from the underlyingsub-surface-level metal feature 105. Although not depicted in FIG. 2B,it should be appreciated that a pad recess disposed over the via 231(e.g., similar to that depicted in FIG. 4B) may also be formed by theprocess which forms the trace recess 212. To form the trace recess 212and/or a pad recess, laser patterning may be performed, for example withan excimer laser.

Next, at operation 214, a seed layer (e.g., seed layer 215 in FIG. 2B)is deposited. In one embodiment, electroless plating is used to form theseed layer 215. For example, a catalyst, such as palladium (Pd) may bedeposited followed by an electroless copper plating process. In analternate embodiment, a physical vapor deposition (i.e., sputtering)technique is used to deposit the seed layer 215.

The via holes, trace recesses, and/or pad recesses are then filled atoperation 224, for example with an electrolytic plating process. Asshown in FIG. 2B, an electrolytic copper plating process is performed todeposit the fill metal 225. At operation 226, over plated fill metal isthen removed by one or more of, etching, buff grinding,chemical-mechanical polishing, etc. to planarize the fill metal 225. Forexample, chemical, mechanical polishing (CMP) or buff grinding may beused to first planarize the fill metal 225 and then an etch may beemployed to remove any remaining fill metal 225 from the top surface ofthe surface-level dielectric layer 110, thereby delineating an embeddedmetallized via 231 and an embedded metallized trace 232. The embeddedmetallized trace 232 may be a necking trace that is carrying a signalfrom a connection that has been made between the die and substrateelsewhere and therefore may not be bonded directly to the solder on thedie side. As further illustrated, sub-surface-level metal feature 105has sidewalls adjacent to the surface-level dielectric layer 110 inwhich the metallized via 231 and metallized trace 232 are embedded.

For embodiments where the seed layer was electrolessly plated, anycatalyst remaining after operation 227 may be removed at operation 235to eliminate the risk of surface finish metals subsequently plating onthe surface-level dielectric layer 110. Various chemical solutions arecommercially available to remove a catalyst depending on the fill metaland the catalyst employed. For example, as shown in FIG. 2B, a Pdcatalyst 229 may be removed with a commercially available wet chemicaltreatment without etching a copper fill metal 225. For embodiments wherethe seed layer was sputtered, operation 235 may be skipped.

At operation 243, a surface finish metal is formed on all of the exposedsurfaces of the embedded metal features (e.g., top surfaces of embeddedmetallized via 231 and embedded metallized trace 232), forming aprotrusion beyond a top surface of the surface-level dielectric layer110. While the surface finish metal is of a different composition thanthe fill metal 225, a variety of surface finish metal compositions orplated stacks may be employed. In the exemplary embodiment depicted inFIG. 2B, an electroless plating process is used to form the surfacefinish metal 245 comprising at least a layer of nickel (Ni), and mayfurther include additional layers, such as palladium (Pd), and/or gold(Au). In an exemplary embodiment, the surface finish metal 245 includesa 6-8 μm thick nickel layer.

In one embodiment, a catalyst, such as Pd, is formed only on the exposedcopper fill metal 225 (e.g., Pd catalyst will only remain on exposed Cumetal due to differences in pre-treatments and Pd activation steps inthe surface finish process compared to that in seed layer depositionprocess). With no catalyst present on the surface-level dielectric layer110, the surface finish metal may be plated in a self-aligned manner sothat no masking of the fill metal 225 is necessary and therefore noadditional overlay/dimensional error margin is required. With thesurface finish metal 245 only formed on the via, the “top pad” isreferred to herein as “metal defined” rather than photo defined orsolder resist defined. The critical dimension (CD), or minimum lateralwidth, W_(SF), of a surface finish metal at a connection point will thenbe at least as large as the largest diameter, W_(v), of the underlyingembedded metallized via 231 or embedded metallized trace 232, but onlylarger than W_(v) to the extent the surface finish metal 245 undergoeslateral expansion during the plating process. Because the metal traces232 are embedded below the top surface of the surface-level dielectriclayer 110, the surface finish metal 245 does not plate on the sidewallsof the fill metal, allowing a reduction in the space between adjacentembedded metallized traces 232. In one embodiment where W_(v) isapproximately 60-65 μm, W_(SF) will be anywhere from equal to W_(v) (foranisotropic plating) to less than W_(v) plus twice the thickness of thesurface finish metal 245 (for isotropic plating). In particularembodiments where connection points have a pitch (i.e., bump pitch) ofapproximately 90 μm and pad size of 63 μm, embedded metal traces 232have a minimum pitch of approximately 9 μm (9 μm smallest laterallydimensioned trace with a 9 μm minimum lateral space to a nearestadjacent metal feature) to provide higher I/O routing density than ispossible with SAP.

As further shown in FIG. 2A, the process 200 may repeat operations 201,209, 211, 214, 224, 227, and 235 to embed metal features in a pluralityof levels and enable multiple I/O routing layers, for example with bothmicro-strip and stripline routing. With multiple layers of embeddedmetallized traces 232, much higher I/O density is possible where I/Orouting is only possible for a surface-level metal layer. Afterperforming the desired number of iterations, the process 200 completeswith the self-aligned surface finish plating operation 243.

FIG. 3A is a flow diagram of a package substrate fabrication process 300for a reduced bump pitch in accordance with an embodiment of the presentinvention. FIG. 3B provides cross-sectional views of selected operationsillustrating stages in the package substrate fabrication process 300, inaccordance with an exemplary embodiment.

As shown in FIG. 3A, at operation 201 a dielectric layer (e.g., thesurface-level dielectric layer 110 in FIG. 3B) is laminated over apatterned metal layer (e.g., sub-surface-level metal feature 105) insubstantially the same manner described in FIG. 2A. At operation 209,via holes (e.g., via holes 210 in FIG. 3B) are laser drilled into thedielectric layer (e.g., surface-level dielectric layer 110) to expose aportion of the underlying metal layer (e.g., sub-surface-level metalfeature 105).

At operation 303, a permanent photodefinable layer (e.g., 305 in FIG.3B) is laminated or coated. Any material suitable for applications suchas controlled collapse chip connect (C4) applications may be utilized asthe permanent photodefinable layer 305. For example, the permanentphotodefinable layer 305 may be a photodefinable polyimide. At operation307, the permanent photodefinable layer is exposed and patterned usingany conventional lithographic technique to form patterned recesses, suchas the trace recess 212 and pad recess 223, as illustrated in FIG. 3B.In the exemplary embodiment, the trace recess 212 has CD of about 5 μmwhile the pad recess 223 has a CD of about 75 μm with the embeddedmetallized via 231 having a maximum diameter, W_(v), of about 50-55 μm.

Operations 214, 224, 227 and 235 are then performed in the same mannerdescribed elsewhere herein for the process 200, but with planarizationand/or etchback of the fill metal 225 at operation 227 exposing a topsurface of the permanent photodefinable layer 305 and delineating anembedded metallized via 231, an embedded metallized top pad 233, and anembedded metallized trace 232. As illustrated, unlike for SAPtechniques, sub-surface-level metal feature 105 has sidewalls adjacentto the surface-level dielectric layer 110 in which the metallized via231 is embedded. Also, because at least some of the permanentphotodefinable layer 305 is permanently retained in the packagesubstrate (some of the photo-definable layer may be removed if a CMPplanarization method w/out selective etchback was employed) any catalyst(e.g., Pd) utilized to form the seed layer 215 on the sidewalls of thetop pad 233 will remain at the interface between the top pad 233 andsurface-level dielectric layer 110. Catalyst 229 on the top surface ofthe permanent photodefinable layer 305 may or may not require removal bya wet chemical treatment prior to surface finish plated depending on theprocess employed to expose the top surface of the permanentphotodefinable layer 305. For example, use of CMP to expose the topsurface of the permanent photodefinable layer 305 may also adequatelyremove any catalyst from the top surface.

As further shown in FIG. 3A, the process 300 may repeat operations 201,209, 303, 307, 214, 224, 227, and 235 to embed metal features in aplurality of levels and enable multiple routing layers, for example withboth micro-strip and stripline routing. With multiple layers of embeddedmetallized traces 232, much higher I/O density is possible where I/Orouting is only possible in a surface-level metal layer. Afterperforming the desired number of iterations, the process 300 completeswith the self-aligned surface finish metal plating operation 243.

Following the removal of any catalyst used to form the seed layer 215from the top surface of the permanent photodefinable layer 305, thesurface finish metal 245 is plated at operation 243. In an embodiment,the plating is self-aligned as limited by the exposed area of copper (atthe embedded metallized top pad 233, embedded metallized via 231, orembedded metallized trace 232). Because sidewalls of the embeddedmetallized trace 232 are not exposed, plating of the surface finishmetal 245 is more anisotropic allowing minimum spaces between adjacentembedded metallized traces 232 and embedded metalized top pads 233adjacent to metalized traces 232 to be, for example approximately 5 μmfor a 1:1 line:space pitch (in the case of a 90 μm bump pitch with 75 μmpads) even when the surface finish metal thickness is greater than atleast half minimum space between adjacent embedded metal filled features(e.g., greater than half the minimum space between adjacent embeddedmetallized traces 232). The permanent photodefinable dielectric 305enables formation of a top pad 233 disposed above the metallized via231. In one exemplary embodiment, the top pad 233 has a maximum lateraldimension greater than a maximum diameter of the via, W_(v).

FIG. 4A is a flow diagram of a package substrate fabrication process 400for a reduced bump pitch in accordance with an embodiment of the presentinvention. FIG. 4B provides cross-sectional views of selected operationsillustrating stages in the package substrate fabrication process 400, inaccordance with one exemplary embodiment.

Process 400 begins at operation 201 with lamination of a dielectriclayer (e.g., surface-level dielectric layer 110 in FIG. 4B) over apatterned metal layer (e.g., sub-surface-level metal feature 105) in asubstrate build-up layer 101. As described elsewhere herein, thepatterned metal layer and any number of layers below the patterned metallayer may be formed in any manner known in the art (e.g., with an SAPtechnique). The dielectric layer may be of any composition known in theart and applied over the patterned sub-surface level metal layer in anyconventional manner. In a particular embodiment, the dielectric layer110 comprises a polymer (epoxy based resin) with silica filler toprovide suitable mechanical properties that meet reliabilityrequirements of the package. The material is also to have a suitableablation rate to enable laser patterning as described elsewhere herein.

Returning to FIG. 4A, at operation 209 via holes (e.g., via holes 210 inFIG. 4B) are laser drilled into the dielectric layer (e.g.,surface-level dielectric layer 110) to expose a portion of theunderlying metal layer. Any conventional technique may be used, such asone employing CO₂ laser. At operation 211 the dielectric is then furtherpatterned to form recesses. FIG. 4B illustrates a trace recess 212 andpad recess 233, each with a depth into the surface-level dielectric thatis less than that of the vias (i.e., the sub-surface-level metal feature105 is not exposed by the patterning at operation 211). For the sake ofsuccinct illustration, both a wide via embodiment and a metallized toppad embodiment is represented in FIG. 4B, but it is to be appreciatedthat wide via and metallized pad embodiments need not be mixed within asingle package substrate and rather one or the other is typicallyemployed in a given substrate. As in process 200, the patterned recessesmay be formed by laser patterning.

Next, at operation 214, a seed layer (e.g. seed layer 215 in FIG. 24) isdeposited. As in process 200, electroless plating techniques may be usedto form the seed layer 215. For example, a catalyst, such as Pd, may bedeposited followed by an electroless copper plating process. In analternate embodiment, a physical vapor deposition (i.e., sputtering)technique is used to deposit the seed layer 215.

The via recesses, top pad recesses, and trace recesses are then filledat operation 224, for example with an electrolytic plating process. Asshown in FIG. 4B, an electrolytic copper plating process is performed todeposit the fill metal 225. At operation 226, overplated fill metal isthen removed by one or more of, etching, buff grinding,chemical-mechanical polishing, etc. to planarize the fill metal 225. Forexample, CMP may be used to first planarize the fill metal 225 and thenan etch may be employed to leave only a thin metal seed layer 215 on thetop surface of the surface-level dielectric layer 110 for subsequentelectrolytic plating of a surface finish metal. As illustrated,sub-surface-level metal feature 105 has sidewalls adjacent to thesurface-level dielectric layer 110 in which the metallized via 231,metallized trace 232, and top pad 233 are embedded.

As further shown in FIG. 4A, the process 400 may repeat operations 201,209, 211, 214, 224, 227 to embed metal features in a plurality of levelsand enable multiple routing layers, for example with both micro-stripand stripline routing. With multiple layers of embedded metallizedtraces 232, much higher I/O density is possible than where routing isonly possible for a surface-level metal layer. After performing thedesired number of iterations, the process 400 completes with the finishmetal plating operation 420.

At operation 420 a dry film resist (DFR) is laminated and patternedusing any technique known in the art. In the exemplary embodimentillustrated in FIG. 4B the DFR 421 is patterned to expose the fill metal225 in the embedded metallized via 231 (and/or embedded metallized toppad 233) without exposing the embedded metallized trace 232. To accountfor alignment variation, openings in the DFR 421 may have smallerlateral dimensions to form a surface finish with lateral dimensionW_(SF) that smaller than the embedded metal filled features (e.g.,W_(TP)).

At operation 445, a surface finish metal is plated over the exposed viafill metal to a thickness less than that of the DFR. The surface finishmetal may be any of those described elsewhere herein, and for examplemay include a stack of Ni, Pd, and Au or Ni and Au metal layers. Asshown in FIG. 4B, the finish metal 425 plates on regions of theremaining seed layer 215 which are not protected by the DFR 421. Becausethe embedded metallized trace 232 is protected by the DFR 421, nosurface finish metal is formed. Minimum spaces between adjacentmetallized traces 232 are therefore rendered independent to the metalfinish thickness.

Optionally, additional fill metal (e.g., copper) may also be plated atoperation 445 prior to plating of the surface finish metal. For anembedded metallized via 231 with a diameter of approximately 50-55 μm, alarger top pad opening (e.g., 75 μm) may be opened and an additionalfill metal and/or surface finish then plated to a thickness less thanthat of the DFR 421 over the seed layer 215 present beyond the embeddedmetallized via 231. For such an embodiment, the fill metal forms atopographical feature (i.e., a top pad) protruding from the top surfaceof the surface-level dielectric layer 110 with sidewalls which are freeof the surface finish metal 245. As such, a top pad may be embedded inthe surface-level dielectric layer 110 (e.g., as illustrated in FIG. 4Bby embedded top pad 233) or disposed above a top surface of thesurface-level dielectric layer 110.

At operation 422, the DFR is removed using any conventional stripprocess and the thin seed layer 215 is selectively etched to expose thesurface-level dielectric layer 110 between the embedded metallized via231, the embedded metallized trace 232, and the top pad 233.

FIG. 5 is a flow diagram of an assembly process 500 utilizing a packagesubstrate with reduced bump pitch in accordance with an embodiment ofthe present invention.

Assembly process 500 begins at operation 501 with receiving a packagesubstrate with exposed surface finish at predetermined substrateconnection points. As such, in the exemplary embodiments, no solderresist is present on the surface of the package substrate and no solderis placed on the surface finish prior to coupling a chip to the packagesubstrate. Any of the package substrates fabricated in the processes200, 300 or 400 may be used in the assembly process 500. FIGS. 6-7 arecross-sectional views of selected operations illustrating stages in thepackage assembly process 500, in accordance with another exemplaryembodiment. As depicted in FIGS. 6-7, the substrate connections pointsmay be either a surface finished pad 233 or embedded metallized via 231.

At operation 502, an IC chip is received with solder bumps disposed onthe chip connection points. While the IC chip may generally be of anyconventional type, in a particular embodiment, the IC chip is amicroprocessor having a large I/O count. In an embodiment, the chip I/Oand power solder bumps may have a pitch of less than 100 μm (e.g., 90μm). As shown in FIG. 6, the IC chip 502 includes solder bump 504 on asurface level chip-side metal feature 503. The IC chip 502 then alignedwith the surface finished substrate to have the soldered IC chipconnection points aligned with the surface finished substrate connectionpoints. The chip side solder is then alloyed at operation 520 to affixthe chip to the substrate connection points. As shown in FIG. 7, thepackage substrate has exposed surface finish metal 245 at substrateconnection points forming a topographical feature protruding beyond atop surface of the surface-level dielectric layer 110 disposed betweenadjacent connection points. In particular embodiments, as describedelsewhere herein, the surface finish metal 245 has a thickness that isequal to at least half a smallest lateral space between the adjacentmetal traces 232.

As shown in FIG. 7, the chip solder bump 504, when alloyed may wraparound the surface finish protruding from the surface-level dielectriclayer 110 to contact a sidewall of the surface finish metal 245. Asillustrated in FIG. 7, the chip solder bump 504 may be coupled directlyto surface finish of the embedded metallized via 231 (i.e., with no toppad of larger dimension than the embedded metallized via 231), oralternatively, the chip solder bump 504 may be coupled directly tosurface finish of the embedded metallized 233.

With the chip affixed to the package substrate conventional underfill650 may be applied to fill the gap between the IC chip 502 and packagesubstrate 101. As no solder resist is applied to the package substrate,the underfill 650 contacts the top surface of the surface-leveldielectric layer 110. For the embodiment illustrated in FIG. 7B, theunderfill 650 also contacts the embedded metal trace 232 where surfacefinish is not applied. The packaging process is then completed usingconventional techniques.

The above description of illustrative embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific implementations of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. The scope of the inventionis to be determined entirely by the following claims, which are to beconstrued in accordance with established doctrines of claiminterpretation.

What is claimed is:
 1. A method of forming an integrated circuit (IC)package substrate, the method comprising: laminating a dielectric layerover a first metal feature; laser drilling a via hole in the dielectriclayer to expose the first metal feature; laser patterning a trace recessin the dielectric layer laterally displaced from the via hole;electrolytically plating a fill metal into the via hole and the tracerecess; planarizing the fill metal to a top surface of the dielectriclayer to form a metallized via and a metallized trace; laminating andpatterning dry resist film to expose the metallized via without exposingthe metallized trace; plating a surface finish metal over the exposedmetallized via; removing the dry resist film; and exposing thedielectric layer between the metallized via and the metallized trace. 2.The method of claim 1, wherein exposing the dielectric layer between themetallized via and the metallized trace further comprises etching away afill metal seed layer formed on the top surface of the dielectric layer.3. The method of claim 1, wherein plating the surface finish metal overthe exposed metallized via further comprises plating one or more ofnickel, palladium, and gold.
 4. An integrated circuit (IC) packagesubstrate, comprising: a first surface-level metal feature embedded in asurface-level dielectric layer with a top surface of a fill metal,formed in the surface-level dielectric layer, planar with a top surfaceof the surface-level dielectric layer; and a surface finish metaldisposed over a top surface of the first surface-level metal feature,the surface finish metal protruding beyond the top surface of thesurface-level dielectric layer and having a different composition thanthe fill metal wherein the surface finish metal has a thickness that isequal to at least half a smallest lateral space between a first andsecond surface-level metal trace embedded in the surface-leveldielectric layer.
 5. The IC package substrate of claim 4, wherein aportion of the first and second surface-level metal traces has nosurface finish metal.
 6. The IC package substrate of claim 5, furthercomprising a sub-surface-level metal feature having sidewalls adjacentto the surface-level dielectric layer.
 7. The IC package substrate ofclaim 4, wherein the first surface-level metal feature has a minimumlateral dimension of 9 μm or less and a smallest lateral space betweenthe first and second surface-level metal traces is no more than 9 μm. 8.The IC package substrate of claim 4, wherein the first surface-levelmetal feature includes a pad disposed above the fill metal, the padhaving a lateral dimension greater than a diameter of the fill metal. 9.The IC package substrate of claim 4, wherein the first surface-levelmetal feature comprises Cu, the surface finish metal comprises nickel,and the surface-level dielectric layer comprises an epoxy based resinwith silica filler.
 10. A package assembly, comprising: an integratedcircuit (IC) chip with input/output (I/O) and power connection points;and a package substrate, comprising: a first surface-level metal featurecomprising a fill metal embedded in a surface-level dielectric layer;and a surface finish metal disposed over a top surface of the fillmetal, the surface finish metal protruding beyond a top surface of thesurface-level dielectric layer, wherein the surface finish metal has adifferent composition than the fill metal, and wherein the surfacefinish metal is affixed to a first of the power connection points bysolder.
 11. The package assembly as in claim 10, wherein the solderwraps around the surface finish metal to contact a sidewall of thesurface finish metal.
 12. The package assembly of claim 10, wherein theIC chip is a microprocessor, wherein the fill metal comprises copper,wherein the surface finish metal comprises nickel, and wherein Pd ispresent at an interface between a sidewall of the first surface-levelmetal feature and the surface-level dielectric layer.
 13. An integratedcircuit (IC) package substrate, comprising: a first surface-level metalfeature embedded in a surface-level dielectric layer with a top surfaceof a fill metal, formed in the surface-level dielectric layer, planarwith a top surface of the surface-level dielectric layer; and a surfacefinish metal disposed over a top surface of the first surface-levelmetal feature, wherein the surface finish metal comprises a metalselected from the group consisting of nickel, palladium and gold, thesurface finish metal protruding beyond the top surface of thesurface-level dielectric layer and having a different composition thanthe fill metal, wherein the first surface-level metal feature includes apad disposed above the fill metal, the pad having a lateral dimensiongreater than a diameter of the fill metal.
 14. An integrated circuit(IC) package substrate, comprising: a first surface-level metal featureembedded in a surface-level dielectric layer with a top surface of afill metal, formed in the surface-level dielectric layer, planar with atop surface of the surface-level dielectric layer; and a surface finishmetal disposed over a top surface of the first surface-level metalfeature, the surface finish metal protruding beyond the top surface ofthe surface-level dielectric layer and having a different compositionthan the fill metal wherein the first surface-level metal featurecomprises Cu, the surface finish metal comprises nickel, and thesurface-level dielectric layer comprises an epoxy based resin withsilica filler.